It is known that loads or electrical networks that are to be protected using automatic circuit breakers comprise electrical conductors corresponding to the phases present, for example three, and possibly the neutral. A circuit breaker for the protection of loads or electrical networks normally comprises devices for making and breaking the main contacts, devices for detection of the currents that traverse it, protection devices, and automatic tripping devices.
The protection devices can normally be of a thermal, magnetic, magnetothermal type, or else of an electronic type.
Operation of protection devices of an electronic type is normally based upon the analysis of signals indicating the currents corresponding to the conductors. Said signals are generated by current-detection devices, technically referred to as current transducers. A low-voltage circuit breaker traditionally uses at least one transducer for each phase, and optionally also one for the neutral. The electronic protection device analyses the signals coming from said transducers to deduce the values of the currents circulating in the conductors. According to said values, the same protection device can generate a command, which, for example via an automatic-tripping solenoid, brings about opening of the contacts of the circuit breaker itself.
Protection devices of an electronic type require electrical supply. For this purpose, the current transducers themselves are usually exploited, but not always with altogether satisfactory results.
Amongst the transducers present in the known art, there may be cited amperometric transformers (internationally referred to as “current transformers” or CTs), Rogowski coils and Hall-effect sensors.
The most commonly used transducers are CTs, which have the prerogative of enabling provision of the so-called self-supply of the protection device. In fact, the output in the form of current, or amperometric output, generated by them, with the main purpose of making it possible to deduce the currents circulating in the protected electrical circuit, is generally characterized by a level of energy that is sufficient even to supply the protection device and the automatic-tripping solenoid.
A decidedly critical limit of CTs in their dual use as current transducers and power-supply units for supplying protection devices lies in the fact that, for high values of the currents circulating in the conductors which are to undergo measurement, the currents generated in the secondary windings and used for supplying the protection devices can markedly exceed the range of optimal operation of the protection devices, and in particular of the electronic circuits. This fact renders necessary the use of special dissipating means, the overall dimensions, weight and cost of which renders them far from compatible, above all in uses for small circuit breakers.
Another limit of this category of transducers is linked to the so-called phenomenon of saturation inherent in the nature of the ferromagnetic materials forming the cores of the windings. Saturation manifests itself with the decay of the linearity of the signal generated at high values of the currents circulating in the conductors undergoing measurement.
Other disadvantages of current solutions comprising CTs as power-supply units and measurement devices are represented by: the possibility of operating only in alternating current, hence precluding the uses in circuits traversed by a d.c. current or a current at a very low frequency; the considerable size of the devices; the heating that is caused; and the functional rigidity linked to the limited possibilities of calibration.
Solutions that are alternative and already present in the known art use the aforementioned Hall-effect sensors or Rogowski coils. These transducers afford marked accuracy and linearity of response, and hence, in the ultimate analysis, enable greater efficiency of the protection device.
Also these latter solutions present, however, considerable limitations as regards the supply of the electronic protection device. The first limit lies in the fact that the energy associated to the signal generated by them is not generally sufficient to ensure supply of the protection device and of the automatic-tripping solenoid directly. It follows that it is usually necessary to use sources of external supply to supply the protection device; in particular, an intrinsic limit of Hall-effect sensors lies in the fact that said sensors have to be supplied.
Also in the case of circuit breakers and similar switches that use these latter types of transducers, tests have in any case been conducted, and different solutions have been proposed, which in certain cases have demonstrated a good functionality, but in practical use none of them has provided totally satisfactory results, above all from the economic standpoint.